Electrical coupling among neurons is an important feature of the integrative functioning of all nervous systems. We are interested in studying what may be a novel mechanism for synaptic modulation which could result as a consequence of electrical coupling among neurons which release different neurotransmitters. We suggest that modulation of the functional electrical coupling between several presynaptic neurons which release different transmitters onto a common follower cell may change the relative proportion of the two transmitters release, resulting in variations in postsynaptic actions. We will study the inhibitory postsynaptic potentials (IPSPs) made by a group of electrically coupled neurons in the stomatogastric ganglion of the lobster, Panulirus interruptus. The two PD neurons are cholinergic and are electrically coupled to the AB neuron, which current evidence suggests is glutamaterigic. The PD-AB group makes common synapese on a number of follower cells. We intend to use conventional electrophysiological, pharmacological, and biochemical methods as well as the recently devised Lucifer-Yellow cell-kill technique to: a) verify (or disprove) that the PD-IPSPs are cholinergic and the AB IPSPs are glutamaterigic b) establish which of the PD-AB/IPSPs are due to the action of transmitter released by both PD and AB neurons c) characterize completely the postsynaptic ionic mechanisms activated by PD and AB released transmitters d) determine if synaptic inputs which modify the electrical coupling, as measured in the cell somata, also modify the electrical coupling of the presynaptic release sites e) determine if the proportion of the PD and AB derived transmitter is affected by modifications in the functional electrical coupling. This hypothesis predicts that under some conditions the PD-AB IPSP would be predominantly cholinergic, under some conditions predominantly glutamatergic, under other conditions mediated by both transmitters. Thus, in any nervous system, including the human, physiological conditions or pharmacological agents which affect electrical coupling among neurons could have important consequences on chemical synaptic transmission